1. Field of the Invention
The present invention relates to a crystal oscillator.
2. Description of the Related Art
A crystal oscillator is an oscillator having a configuration in which a quartz resonator, a CMOS inverter, a capacitor, a resistor, and a CMOS buffer are encapsulated in a package.
An example of a conventional crystal oscillator will be described with reference to FIG. 4.
A crystal oscillator 10 includes an oscillation circuit part 1, a quartz resonator 2, and a CMOS buffer 6 serving as an output circuit part, and the oscillation circuit part 1 includes a resistor 4, a CMOS inverter 5, and capacitors 8a and 8b. Thus, the crystal oscillator 10 has a circuit configuration that a general crystal oscillator has. An output terminal 7 is connected to an output part of the CMOS buffer 6. Quartz resonator measurement terminals 3a and 3b are connected to both ends of the quartz resonator 2, respectively.
A power supply (not shown) is connected to the oscillation circuit part 1 and CMOS buffer 6, and supplies a predetermined voltage to the oscillation circuit part 1 and CMOS buffer 6 for their operation.
Generally, a crystal oscillator does not oscillate when a resistance value of the quartz resonator 2 is larger than a negative resistance value of the oscillation circuit part 1. Thus, in order to stably continue oscillation of the crystal oscillator, it is important to reduce the resistance value of the quartz resonator 2 and to increase the negative resistance value of the oscillation circuit part 1. As a value indicating whether or not the crystal oscillator stably oscillate, a ratio between the negative resistance value of the oscillation circuit part 1 and the resistance value of the quartz resonator 2 has conventionally been used as “oscillation margin”. Experiments reveals that stable oscillation can be obtained when a value of the oscillation margin is 4 to 5 or more.
There is a method that checks the oscillation margin of the crystal oscillator. In this method, the resistance value of the quartz resonator 2 is measured at the quartz resonator measurement terminals 3a and 3b which are exposed outside the package, and it is checked whether or not the measured value is sufficiently small.
Further, as a method for checking the oscillation margin of a crystal oscillator which has been incorporated in a device, JP 2007-116563 A discloses a technology in which a check resistor is connected in series between a quartz resonator and an oscillation circuit, and it is checked whether the crystal oscillator normally oscillates or not by switching connection to the check resistor in response to a control signal from outside.
JP 2001-94347 A discloses a technology in which a variable resistor is connected in series between a crystal current control circuit and a quartz resonator, and a resistance value at which oscillation is stopped is measured by varying the crystal current and the variable resistor.
JP 2010-246059 A discloses a technology that calculates a series resonance resistance value of the quartz resonator from the negative resistance value obtained when oscillation is stopped or started by changing the negative resistance value of the oscillation circuit part of the crystal oscillator using a control signal from outside.
In the method that measures the resistance value of the quartz resonator 2 at the quartz resonator measurement terminals 3a and 3b exposed outside the package, measurement is made at a specified drive level, so that it is difficult to check the resistance value during normal operation of the crystal oscillator. Further, the resistance value of the quartz resonator 2 may change in characteristics depending on a condition of foreign matters adhered thereto or due to influence of heat treatment, so that it is difficult to grasp a change in state after inspection time, even if no abnormality has been founded at the inspection time. Furthermore, although this method can be easily applied to a single crystal oscillator, the measurement may be difficult to perform after the crystal oscillator is incorporated in a system.
In the technologies disclosed in JP 2007-116563 A, JP 2001-94347 A, and JP 2010-246059 A, the measurement needs to be performed in a state where operation of the crystal oscillator to be measured is stopped. Thus, execution of the measurement may lead to stop of a part of or the entire system. Particularly, the measurement is difficult to perform when a crystal oscillator to be measured is used as a clock of a controller that measures the oscillation margin.